flood frequency analysis based on hybrid regression method for providing regional flood model (case study: khorasan razavi province)

in regions with deficient or no data, it is necessary to apply indirect methods to estimate the peak flow. regional flood analysis is the most accurate and reliable technique in such cases. one of the regional flood methods for flood analysis in arid and semi arid regions is hybrid technique, which was applied on data of khorasan razavi province watersheds to estimate the peak flow. hybrid method based on station - year method which considers available statistics from all stations to overcome statistics shortage problem. hybrid method consists of two sections. at first, the most important parameters including area, annual rainfall and height are determined using factor analysis. annual peak flow was standardized based on an approximate factor and then approximate standard factor was improved based on combined regression and analysis frequency. the results showed that the lowest relative error for the models was related to 50-year return period while relative error increased in other return period. as the relative errors were 1.03 and 0.79, respectively in the return periods of 2 and 100 years. therefore, according to results, hybrid method is suitable for return periods 20 to 50 years in the study area.